MotoTune 14

An introduction to induction kits

noreply@blogger.com (mudhy) — Thu, 24 Feb 2011 22:26:00 -0800

As induction kits are probably one of the most popular modifications for car tuning projects we thought we should add a introductory article to explain what they are, what they do and the theory behind them.
An induction kit is quite simply an air filter. Most cars have air filters mounted in an air box.
These filters are generally constructed of paper which filters out the particles from the air and prevents these from getting into the engine. The airboxes are designed to reduce the noise of the engine and have various vanes and angles cut to maximise this noise reduction.
Because the air is not flowing freely through the filter there is potentially a loss of power due to the turbulence caused by all that sound deadening. To make up for this loss of power the air filter surface area is a lot larger than the intake tubes so manufactures have effectively closed the gap between, noise reduction and performance.

When it comes to car tuning noise reduction is generally the last thing on peoples mind. Swapping out the restrictive air box with an open filter helps to improve the flow of air into the engine and fully releases the induction roar, a noise associated with tuners cars the world over.

The induction kit is quite simply an open air filter shaped like a cone, dome or tube.

So it would appear that induction kits are a win/win situation. However there are some downsides. As the filter is sited in the warm engine bay it will suck in warm air which carries less oxygen. Thankfully after a mile or so the air in the engine bay is much reduced and closer to ambient temperatures. Fitting a cold air feed pipe taking air from outside the engine bay will help to counter this problem.
Engines with induction kits fitted typically see the power gains quite high up the RPM range. Low down power can be compromised. In small engines the effect of an induction kit is a noticeable loss of low end power. This is fine if you want to use the top end of the RPM range all the time, like you would in a race environment but on the street this is just not viable. Fuel usage would be much higher and you will be causing a noise nuisance to your neighbours.
Induction kits flow better than paper elements partly because they do less filtering. Induction kits are still efficient filters but they are never going to be as effective as an OEM paper filter.
Do inductions kits always add power?
NO - typically power gains are toward the top end of the RPM range and some engines will show a power loss, especially low down.
Do induction kits add fuel economy?
By sucking in warm engine bay air the engine will use less fuel, so round town you might notice a slight improvement in MPG but the induction roar promotes a heavier driving style which negates this.
Are all induction kits the same?
No, quality varies considerably. We recommend filters that are using a cotton gauze filtration medium. Some kits come with high flow air boxes and air intakes designed specifically for a car and these will usually out perform basic filter only kits.

Are induction kits hard to fit?
Induction kits are very simple to fit. You may need to cut a pipe or two but most kits can be fitted in 4 minutes with just a screwdriver.
I fitted an induction kit and I have flat spots?
Wait for a while. Modern engines may need to adjust to the new air flow characteristics. They will auto trim the fuelling based on lambda readings. If after a tank of fuel you still get flat spots then you should check your fuel delivery. Running lean can be a problem and some have noted that fitting a sports exhaust will further improve things.
What is the main benefit of an induction kit?
If you are looking for more power then there are lots of better things to spend your money on. If you want the induction roar noise then the induction kits are good value.

SpeedFactory has created the fastest Dodge Charger in the world!

noreply@blogger.com (mudhy) — Thu, 24 Feb 2011 22:16:00 -0800

The American tuners at SpeedFactory have developed a modified version of the Dodge Charger 426 Hemi that can cover the quarter-mile (402 m) in 9.453 seconds. This is the fastest model built on a Chrysler LX/LC platform in the world. This platform is used by the Dodge Charger Magnum and Challenger but also by the Chrysler 300C.
The American tuners claim that they have established a new world record with this model.
The SpeedFactory version of the Dodge Charger 426 is equipped with a V8 Hemi engine which has a Vortech V7 YSI compressor and develops a staggering 900 bhp.The interior of this model has completely removed to reduce the weight of the Charger to 1.700 kg. The interesting fact about this model is that it keeps the best part of the the standard elements of the Challenger, the only upgrades being made to the differential and the transmission, which can cope with up to 1.000 bhp. The mechanical compressor that is installed on this model can cope with up to 1.200 bhp with a 78% efficency.
The SpeedFactory tuners haven’t yet revealed the price for this performance package. The model has also received a reinforced suspension which helps the driver to kept the trajectory in the extreme drag races. The record of the SpeedFactory Dodge has been recorded on the South Georgia Motorsports Park track, the speed at the end of the quarter-mile track was 145.3 mph (233.9 km/h)

Porting and Cylinder Scavenging part.1

noreply@blogger.com (mudhy) — Thu, 24 Feb 2011 21:12:00 -0800

TODAY, when we take a look down the cylinder of a two-stroke engine, we find
its walls literally filled with ports to handle the induction, transfer and exhaust phases
of gas flow through the engine.
Those of us who have grown up in the Japanese twostroke
era take it for granted that every cylinder has a huge exhaust port flanked by
anything from four to six transfer ports' However, it hasn't always been this way. As
far back as 1904 Alfred Scott patented his original two-stroke vertical twin. Then in
1906 the French Garard motor appeared with a rotary disc inlet valve. Scott also
developed a rotary valve engine in 1912, winning the Senior TT in that year and the
following year. However in spite of some very innovative designs being incorporated in
two-stroke engines they continued to be embarrassingly unreliable and this single
factor stifled development right up until the time of World War II.
In the mid-1930s, the DKW company set out to make two-strokes respectable.
They were in the business of manufacturing economical two-stroke motorcycles and
stood to profit from changing the two-stroke's image. They engaged the services of an
engineer named Zoller to build a 250 racer, which ultimately won the Isle of Man TT in
1938. This led to the development of a 125 single employing a porting arrangement
originally invented for two-stroke diesels by German engineer Dr.E.Schneurle. It was
this concept which ultimately brought success to the two-stroke, both as an economical
power source for transport and as a powerful, light-weight power source for
competition. Schneurle's loop-scavenging method, patented in 1925, employed a single
exhaust port flanked by two small scavenge or transfer ports, whose air streams were
aimed to converge on the cylinder wall opposite the exhaust (FIGURE 3.1). Being
aimed away from the exhaust, the transfer streams had a natural resistance to shortcircuiting
straight out the exhaust. Earlier designs had used deflector-dome pistons to
keep the fuel/air charge away from the exhaust port. This increased the piston's heat
gathering area and meant that only low power outputs could be aimed for without
continually risking piston seizure.
After the war DKW moved to Ingolstadt in West Germany, while their old plant atZschopau in East Germany was rebuilt as Motorradwerke Zschopau, or MZ. In 1952
Walter Kaaden joined MZ to take over development. His early work concentrated on
exhaust development and alternate scavenge methods. After much experimentation he
proved that the Schneurle loop-scavenge system yielded the best power and reliability.
Then in 1957 he added a third transfer port, opposite the exhaust. Its air stream joined

with the two main transfer ports, directing flow up toward the head (FIGURE 3.2) Contemporary two-stroke technology was introduced initially to Suzuki, and later
to Yamaha in Japan when Ernst Degner defected from East Germany to join Suzuki.
By combining designs which Degner brought from MZ with Japanese technology in the
field of metallurgy two-stroke power outputs and reliability took a leap forward.

The Cylinder Head part.4 (Compression Ratio)

noreply@blogger.com (mudhy) — Mon, 14 Feb 2011 22:47:00 -0800

As the Japanese motorcycling industry has been working with expansion chambers
as long as most, I am sure they are aware of the fact that the return wave actually forces As this bike is designed to run on 90-95 octane fuel the compression can safely be
bumped up to 13.7:1 for competition use on 100 octane racing fuel.
After all this confusion over all sorts of compression ratios you are probably
bewildered and wondering what compression ratio your motor will be safe on.
If you
take a look at TABLE 2.2, you will find the answer.

These figures are what I consider
to be the maximum safe compression ratio for competition engines with good air cooling and an operative squish band. Water-cooled engines will generally run 0.5-1
ratio higher than indicated and motors grossly oversquare for their capacity i.e.,
Yamaha IT 175 usually require a number 0.5-1 lower than shown. Engines running a
pump fuel should not have the compression ratio pushed higher than the manufacturer
has specified. Most companies set their off-road engines up to run on 90-95 octane
petrol (gasoline) but some, like the Rotax engines used in the SWM and Can-Am bikes,
are really only happy on 100 octane. Road racing engines require 100 octane fuel on
standard compression.To keep all that tightly compressed fuel/air charge in the engine there must be a
perfect seal between the head and barrel. If you are really desperate, you could weld the
head to the barrel like they do in the twin turbo Le Mans Porsches, but the the more
usual method is to use an annealed copper or aluminium head gasket inserted between
the two.
McCulloch go-kart engines have used a thin 0.4mm aluminium head gasket for
years without too many problems, providing the engine remains stock, but I can't
really recommend aluminium for any other two-stroke as there always seems to be a
problem with leakage. McCulloch experienced a reverse problem in that they had used
aluminium gaskets for years then, on the MC-92 motor, they switched to copper. The
copper gasket always seemed to leak as you couldn't get the head tension high enough
to crush the copper gasket without the head distorting. On the new MC-93 they have
reverted back to the old aluminium gasket.
When a copper gasket is used, there is always a temptation to reuse the old one.
My advice is don't, unless the gasket has exactly the same inside and outside diameter
as the top lip of the barrel. Even then the gasket should be heated with a low heat gas
flame and allowed to cool and anneal.
Generally, I prefer to run air-cooled motors without a head gasket if the head is
recessed to give a spigot fit with the top of the barrel. In this instance I lap the head
onto the barrel, using valve grinding paste. When you are finished, be very careful to
get all traces of paste out of the cylinder and then clean the head and barrel so that the gasket sealant will take. Remember that removing the head gasket will raise the
compression ratio significantly.
Regardless of the type of gasket used, or even if you choose not to use a gasket, I
recommend the use of either Permatex No.3 or Hylomar SQ-32M gasket sealant. Both
sealants will provide a good seal at the elevated temperatures experienced in two-stroke
engines.

Two-stroke cylinder heads are easily distorted, so you have to be very careful not
to tension the studs more than recommended. Overtensioning will always cause the
head to warp. You must be careful to tighten the studs in at least three progressive
steps. If the head required 20ft/lb tension, you should take all the nuts down finger
tight and then to 10, 15 and finally 20ft/lb. After about 15 minutes go over the nuts
again and then, after the motor has cooled from a run for a minimum of one hour,
tension the nuts again.

The Cylinder Head part.3 (compresion)

noreply@blogger.com (mudhy) — Sun, 13 Feb 2011 21:25:00 -0800

Heat is the enemy of two-stroke engines and stretching the compression ratio to
give a 10% power increase will possibly result in a 3% power rise at the most; the rest
will be lost in heat energy and pumping losses. However, at lower engine speeds the
cylinder will not be completely filled with fuel/air mixture and the power may jump by
5-6% because there is not such a heat loss.
This is, in fact, the real benefit of raising the
compression ratio, not to increase maximum power but to pick up mid-range power
and possibly widen the power band.
Because so much confusion exists in the motorcycle industry relating to
compression ratio we need to define exactly what we are talking about when we use the
term. Ever since the first internal combustion engines, regardless of whether the
engines were two-stroke, four-stroke, diesel, petrol, etc., compression ratio was taken
to mean the ratio of the volume of the cylinder with the piston at BDC to the volume of
the cylinder with the piston at TDC (FIGURE 2.6).

This relationship is expressed in the
formula:

CCV, combustion chamber volume, is made up of the volume of the combustion
chamber, plus any space existing between the piston crown and the top of the cylinder,
plus the head gasket. This volume can be worked out geometrically but it is much
simpler to bring the piston up to TDC. Seal around the edge of the piston with a thin
layer of grease. Fit the head and head gasket and measure the volume with water or
paraffin, using a burette graduated in 0.lcc.
As an example of how these formulas work we will consider the long stroke
Bultaco Pursang 125. This engine has a bore of 51.5mm, a stroke of 60mm and,
according to the manufacturers, a compression ratio of 14:1.

Therefore the engine has a compression ratio just a touch lower than specification.

As this engine will be running at the speedway using 110 octane fuel (Avgas 100/130)

the compression ratio will be increased to 15:1. The standard motor is designed to run

on 95 octane fuel.

The formula to find the required combustion chamber volume is:-

Therefore the combustion chamber volume must be reduced by 9.8 - 8.93 = 0.87cc

To find how much the head must be skimmed to reduce the volume by 0.87cc we

use the cylinder displacement formula transposed to read:-

The above compression ratio is now referred to as the uncorrected compression

ratio. The Japanese have introduced a new way of measuring the compression ratio,

called in various circles effective, corrected, actual or trapped compression ratio. This

can be very confusing because an 8:1 corrected compression ratio is about equivalent to

a 15:1 compression ratio calculated by the old method.

The Cylinder Head part.2 (squish clearance)

noreply@blogger.com (mudhy) — Thu, 10 Feb 2011 21:19:00 -0800

If your machine is only for play, and that's the use to which many motocross bikes
are put, a wide squish clearance will not matter. You will not get peak power, but you
will possibly never know the difference. And you will probably never ride hard enough
to experience detonation.
However, if you want top power and no risk of detonation, the squish clearance
must be closed up. A squish band that isn't working is worse than no squish band at all
as it wastes part of your fuel/air charge. Wasted fuel charge spells less horsepower.

To give you an idea of how much horsepower you could be losing it would be good
to consider the example of a TZ250 Yamaha road racer. These engines have a bore
54mm in diameter and an offset squish chamber. The compression ratio uncorrected is
about 15:1, meaning that the trapped charge is compressed into a space 8.8cc in
volume. If the squish clearance is 1.7mm (lots of motors come from the factory like
that) 1.94cc of the trapped charge will not be burned until well past TDC, too late to
produce any power. 1.94cc represents 22% of the inlet charge lost. When the squish
clearance is reduced to 0.8mm the charge loss is reduced to 0.92cc or 10.5%. On paper
it would seem an easy way to pick up 11.5% more power, but losses reduce this increase
to about 5-6% on the dyno. Therefore maximum power goes up from 52 to 55 hp. Midrange
power can rise as much as 10%, so the bike is easier to ride and it doesn't
detonate.
Reducing the squish clearance is not easy, you can't just machine 1mm, or
whatever, off the head as the compression ratio would end up many numbers too high.
Also you must be sure not to reduce the clearance so much that the piston will bang
into the head at high rpm. The clearance required will vary from engine to engine, and
also on how careful you intend to be each time you replace a piston, rod or barrel.
Pistons usually vary in compression height by up to 0.2mm. Conrods are supposed
to be within a 0.2mm range but they can be up to 0.5mm out. Cylinder heights are
maintained within 0.4mm. In the worst case you could rebuild the motor with a new
piston, rod and barrel. The piston could be 0.2mm taller and the rod 0.2mm longer.
Together with a cylinder 0.4mm shorter than before, the new parts could reduce your
squish clearance by 0.2 + 0.2 + 0.4 = 0.8mm which would result in a blown motor if the
clearance was set at 0.8mm previously. Manufacturers realise this, so they purposely
set the clearance wide to make allowance for the worst possible parts size combination.
If you are willing to measure the squish clearance each time you do a rebuild, and
then compensate for inadequate clearance by fitting a thicker barrel base gasket or a
thicker head gasket, you can reduce the clearance down to the amount shown in

TABLE 2.1.
To find accurately what the squish clearance figure is, the barrel must be tensioned
down on a standard thickness base gasket. Clean all traces of carbon from the head and
piston. Place a strip of modelling clay 20mm wide by 3mm thick across the pistoncrown. Fit the head gasket and head and turn the crank to move the piston just past
TDC. Remove the head and then cut the clay down the middle with a sharp, wet knife.
Carefully pull one strip of clay off the piston and then measure the thickness of the clay
left on the piston. You have to be accurate, so use the end of your vernier calipers. As a
cross-check also measure the clay thickness on the other side of the piston. If the
thicknesses vary this would indicate that the head gasket surface has been machined on
a different plane to that of the combustion chamber. Also at this time measure and
record, for future reference, the compressed thickness of the base gasket and head

gasket. (FIGURE 2.3).

After the clay thickness is measured you can work out how far the head must be
machined to give the desired squish clearance. As mentioned previously, the combustion
chamber must also be machined deeper into the head to keep the compression ratio at
an acceptable level. If you wish to keep the compression ratio the same as standard, the
combustion chamber will have to be machined twice as deep as the amount skimmed
off to reduce the squish clearance, assuming a 50% squish band. Therefore if 0.9mm is
removed, the combustion chamber will have to be made 1.8mm deeper. A 50% squish
band is one having an area equal to half the cylinder bore area ie. an engine with a

54mm bore would have a squish band approximately 8mm wide (FIGURE 2.4).
To check that the machine shop recuts the combustion chamber to the originalcontour when it is deepened, you will have to make a template of the chamber shape
before you send the head off. The template can be made out of any light gauge metal or
even stiff cardboard. (FIGURE 2.5).

Most people like to see the compression ratio pushed up as high as possible. High
compression has always been equated with high horsepower. I agree that the
compression ratio should be made as high as practicable, but often the manufacturer
has already found the limit and built his engines accordingly. All you can do in this
instance is check that production tolerances have not lowered the ratio significantly
below that which the manufacturer intended.
Something you must always remember when dealing with two-stroke engines is
that increasing the compression ratio will not give a power gain equivalent to that
which you would pick up with a four-stroke engine.

The Cylinder Head part.1 (squish chamber)

noreply@blogger.com (mudhy) — Thu, 10 Feb 2011 20:54:00 -0800

THE TWO-STROKE cylinder head certainly doesn't look very exciting but its
design has a large bearing on how well your engine will run. Manufacturers use various
external shapes and cooling fin patterns but the main requirement here is that the
cooling area be large enough to adequately cool the engine. Some people feel that the
head must have radial fins to be any good, but I disagree. Conventional finning is
entirely adequate. It is the surface area which counts, not the fin pattern.

What is more important is the shape of the combustion chamber and the location
of the spark plug. Over the years many combustion chamber designs have been tried,
but only a couple are conducive to a reliable, high horsepower engine. The one thing a
powerful two-stroke doesn't need is a combustion chamber that promotes detonation,
the killer scourge of all racing two-strokes.
To understand the type of combustion chamber you need it is necessary to
appreciate just what detonation is and what can be done to be rid of the problem.
Detonation occurs when a portion of the fuel/air change begins to burn spontaneously
after normal ignition takes place. The flame front created by this condition ultimately
collides with the flame initiated by the spark plug. This causes a rapid and violent
pressure build-up, and the resulting explosion hammers the engine's internal
components.
Detonation leaves many tell-tale signs for which the two-stroke tuner should have
an ever-wary eye. The most obvious sign is a piston crown peppered around the edge as
though it has been sand blasted. Bikes with plated aluminium cylinders will usually
show the same sand blasted effect around the top lip of the bore. A cracked (not
molten) spark plug insulator also indicates detonation. If kept running, a detonating
engine will eventually seize and/or have a hole punched right through the top of the
piston.
The conditions leading to detonation are high fuel/air mixture density, high
compression, high charge temperature and excessive spark advance. A high piston
crown or combustion chamber temperature can also lead to this condition. In a racingtwo-stroke all of these detonation triggers are virtually unavoidable, with the exception
of excessive spark lead.
Researchers have found that it is the gases at the very outer limits of the
combustion chamber, called the 'end gases', that self-ignite to cause detonation. These
end gases are heated by the surrounding metal of the piston crown and combustion
chamber, and also by the heat radiating from the advancing spark-ignited flame. If the
spark flame reaches the outer edges of the combustion chamber quickly enough, these
end gases will not have time to heat up sufficiently to self-ignite and precipitate
detonation. Herein lies the key to prevent detonation — keep the end gases cool and
reduce the time required for the combustion flame to reach the end gases.
The most obvious step that would satisfy the second requirement is to make the
combustion chamber as small as possible, and then place the spark plug in the centre of
the chamber. Naturally the combustion flame will reach the end gases in a small
combustion space more quickly than if the chamber were twice as wide. Additionally, a
central spark plug reduces flame travel to a minimum. (FIGURE 2.1)
In meeting the second requirement, the need to keep the end gases cool can also be
accommodated. If we move the combustion chamber down as close to the piston crown
as possible, no combustion will occur around the edges of the chamber until the piston
has travelled well past TDC. This large surface area acts as a heat sink and conducts
heat away from the end gases, preventing self-ignition.
The chamber just described is called a squish-type combustion chamber because of
the squish band around its edge. Originally, the squish band was designed to squish the
fuel/air charge from the edges of the cylinder toward the spark plug which, of course,
it still does. The fast moving gases meet the spark plug and quickly carry the
combustion flame to the extremity of the combustion chamber, thus preventing
detonation.
Since that time, more benefits of the squish chamber have come to light. The
mixture being purged across the combustion chamber from the squish band
homogenises the fuel/air mixture more thoroughly and also mixes any residual exhaust
gas still present with the fuel charge. This serves to speed up combustion by preventing
stale gas pockets from forming. Such pockets slow down, and in some instances can
prevent, flame propogation.
Turbulence caused by the squish band also serves to enhance heat transfer at the
spark-initiated flame front. Without proper heat transfer, jets of flame would tend to
shoot out toward the edges of the combustion chamber, prematurely heating the
surrounding gases to start off the cycle leading to detonation.
Rapid combustion has other advantages besides controlling detonation. With an
increase in combustion speed there is, of necessity, a corresponding decrease in spark

advance. The closer to TDC we can ignite the charge, the less negative work we have to
do compressing a burning charge that is endeavouring to expand. Also there is less
energy loss in the form of heat being transferred to the cylinder head and piston crown.
When less heat is conducted to the head and piston, the engine runs cooler and
makes more power. A side benefit resulting from the cooler piston also enhances the
power output. A cool piston does not heat the charge trapped in the crankcase as
much, therefore a cooler, denser fuel/air charge enters the cylinder each cycle, to make
more power.If you think about it, you will see that the compact squish type combustionchamber also contributes to a cool piston by confining the very intense combustion
flame to about 50% of the piston crown just before and after TDC.Engine designers have known about these things for a considerable time. This is

why you will find the best racing engines follow the squish design. Also you will notice

that these engines have a very small bore in relation to their stroke, as this too cuts

down the size of the combustion chamber and reduces the area of piston crown exposed

to the combustion flame.

In an effort to minimise cylinder and piston distortion, some manufacturers have

chosen to use an offset squish type combustion chamber (FIGURE 2.2). The exhaust

side of a two-stroke cylinder and piston is always the hottest, even though cooling air

flow is much better here than on the back (inlet side) of the engine. There are several

reasons for this, all associated with the passage of very hot (630°C) exhaust gas

through the exhaust port. The escaping gas heats the exhaust port and cylinder wall as

well as the side of the piston. This can cause the piston to expand abnormally and in

some circumstances to seize. To take care of this possibility, the manufacturer may

choose to increase piston to cylinder clearance, but this may not be desirable as extra

clearance can increase leakage past the rings and usually results in high piston wear. A

safer step is to move the combustion chamber to the rear of the head. If this is done,

the front of the piston crown is shielded from the combustion flame by the squish

surface. Then, when the front of the piston is heated during the exhaust stroke, it will

not expand so far due to its being initially much cooler.

Several two-stroke engines are produced with squish and offset squish chambers,

but unfortunately mass production usually reduces their effectiveness. It is a very

difficult task to keep tolerances of closer than about 0.2mm in production. Therefore

you find many engines with a squish clearance of 1.3-1.8mm instead of the 0.6-0.8mm

clearance that is required.

TWO STROKE TUNING

noreply@blogger.com (mudhy) — Thu, 10 Feb 2011 20:24:00 -0800

MECHANICALLY, the two-stroke engine is very simple, and unfortunately on

too many occasions this apparent simplicity has fooled would-be tuners into believing

that this type of power unit is easy to modify. Just a few hours work with a file in the

exhaust and inlet ports can change the entire character of the engine for the better, but

if you go just 0.5mm too far, you could end up with a device slower than its stock

counterpart.

Therefore modifications must be planned carefully, keeping in mind that seldom,

if ever, is the biggest (or most expensive) the best. As you plan your modifications

always tend to be conservative. If necessary, you can go bigger later.

Possibly the worst viewpoint you can start out with is that the manufacturer didn't

know what he was doing. I started out thinking that way too; but then I began to realise

why the engineers did it that way. Pretty soon I was learning more about what makes a

two-stroke fire — and making fewer mistakes.

You must keep in mind that all production engines are a compromise, even highly

developed racing engines like the Yamaha TZ250. You can make the TZ churn out

more power, but will you be able to ride it with the power band narrowed right down,

and do you have the experience to handle a sudden rush of power at the top end on an

oily or wet track? Also, think about the added wear caused by more rpm and

horsepower; do you have the finances to replace the crankshaft, pistons and cylinder

more frequently now that you are running at 12,500rpm instead of 11,500rpm? When

you begin to think about things like this, you start to understand a few of the reasons

why manufacturers make compromise engines and machines. Remember the TZ250

started out as a road racer, so you can imagine some of the problems you could come

up against if you were to modify a single cylinder 125 motocross engine for use in a

road racer.

Obviously the first work you should do is bring the engine up to the

manufacturer's specifications. This is termed blueprinting, and involves accurately

measuring everything and then correcting any errors made in production. You will be

amazed at the gains to be made, particularly in reliability, and to a lesser extent in

performance, by correcting manufacturing deficiencies. I am convinced manufacturers

bolt their road racers together merely to make shipping all the pieces easier, such are

their tolerances.

I have seen engines that have never been started with piston clearances larger than

the manufacturer's serviceable limit. Conrods that vary 0.4mm in centre to centre

length and 20 grams in weight, on the same crank. Crankwheels which are 0.1mm

outside true centre. Cylinder heads with a squish band clearance of 1.7mm, instead of

0.7-1.0mm. Cylinders with port edges so sharp that the side of the piston and rings

would have been shaved away in a few minutes' running. New pistons with cracks. New

cylinder heads that are porous.

Included in blueprinting is cleaning the rough cast out of the ports, and matching

all gaskets so they don't overlap the ports. The transfer ports must be matched to the

crankcase. The carburettor should coincide with the mounting flange and inlet port.

Anything the manufacturer has not done (presumably to cut costs), you should do.

Blueprinting is slow, tedious work, and it can be expensive when crankshafts have

to be separated and then machined and trued, or when cylinder heads have to be

machined to close up the squish band without raising the compression ratio. It is not

very exciting work because when you have finished the engine is stock standard, and

telling your mates all the work you have done won't impress them. But don't let this

put you off, the basis for any serious tuning must begin with bringing the engine up to

the manufacturer's specifications.

Most people won't believe how close to standard are the motors used by the

factory racing teams. Other riders are convinced that, because the factory boys are

quicker, they must have more power and lots of trick parts. In truth, the differences are

in frame geometry and the ability of the factory rider to ride faster and make the right

choice of tyres, suspension settings, gearing, jetting etc. Plus, of course, they use

blueprinted engines.

So that there is no misunderstanding of the two-stroke operating cycle I will

describe what goes on in each cylinder, every revolution of the crankshaft.

The first example is the piston-ported Bultaco Matador Mk4 which, like most

modern two-strokes, operates on the loop scavenge principle. As the piston goes up,

the inlet port is opened by the piston skirt at 75° before TDC (Top Dead Centre) and

the atmospheric pressure (14.7 psi) forces air/fuel mixture in to fill the crankcase

(FIGURE 1.1). The piston continues to rise to TDC, compressing the fuel/air charge

admitted on the previous cycle. At 3.2mm before TDC the spark plug fires, sending the

piston down on the power cycle. As the piston continues its descent the inlet port is

closed and the fuel/air mixture is partially compressed in the crankcase. 85° before

BDC (Bottom Dead Centre) the exhaust port is opened by the piston crown and the

exhaust gases flow out. After another 22° (63° before BDC) the blow-down period

finishes and the piston crown exposes the transfer ports to admit the fresh fuel/air

charge. This is forced up the transfer passages due to the descending piston reducing

the crankcase volume by the equivalent of the cylinder displacement, in this instance

244cc. As the piston begins rising, the mixture continues to flow into the cylinder and

the exhaust gas continues flowing out. The piston continues rising, closing off first the

transfer and then exhaust ports. Next the inlet port opens, to start the cycle over again.

10 Rotary valve engines operate on the same loop scavenge principle, but in this case

Introduction

A -mixture in cylinder is compressed & inlet cycle begins.

B-mixture in crankcase is compressed.

C- exhaust cycle begins & primary compression continues.

D- transfer cycle begins & exhaust cycle continues.

Fig. 1 .1 Basic two stroke operation.

a disc partially cut away and attached to the end of the crankshaft opens and closes an

inlet port in the side of the crankcase. The Morbidelli 125 twin road racer is a rotary

valve engine. The inlet port opens 30° after BDC and closes 79° after TDC. The piston

crown opens and closes the exhaust and transfer ports.

The following pages will provide you with the knowledge necessary to develop a

successful two-stroke competition engine, but do keep in mind the principles outlined

in this chapter so that you avoid the most basic pitfalls associated with two-stroke

tuning.